1. Technical Field
The present invention relates to: a radiolucent window, which transmits radiation entering a radiation detector; a radiation detector; and a radiation detection apparatus.
2. Description of Related Art
Some X-ray detectors use a detection element, such as an SDD (Silicon Drift Detector), which requires cooling. For such an X-ray detector, vacuum sealing or dry gas enclosing is performed in order to prevent dew condensation in the detection element. Therefore, an X-ray detector is provided with an X-ray transmissive window, which transmits X-rays and can withstand a differential pressure between inside of the X-ray detector and the atmosphere. Conventional structures of an X-ray transmissive window are mainly divided into two types. A first type of an X-ray transmissive window is a thin film (having a thickness of several μm to several tens μm) made of a material, which satisfactorily transmits X-rays and has high intensity and high toughness. The material to be used is a metal, such as beryllium, which has a small atomic number. A second type of an X-ray transmissive window is obtained by holding an organic film, which is made of polyimide or the like, or a CVD (Chemical Vapor Deposition) film (having a thickness of several tens μm to several hundreds μm), which is made of diamond or Si3N4 or the like, with a rib, which is made of silicon or diamond and has a beam structure (having a height of several hundreds μm).
The first type of an X-ray transmissive window has a large thickness and thus hardly transmits X-rays of 1 keV or lower. The second type of an X-ray transmissive window is superior in that a part having no rib can transmit even X-rays of 1 keV or lower. However, in a case where an X-ray enters obliquely with respect to an X-ray transmissive window, the X-ray is blocked by a rib and cannot be transmitted through the X-ray transmissive window. U.S. Pat. No. 7,737,424 discloses an X-ray transmissive window wherein the probability that an X-ray is blocked is lowered by devising the form of a rib so that the aperture ratio with respect to the incident direction of X-rays becomes higher. Regarding a conventional X-ray detection apparatus, an X-ray emission source and an X-ray detector are located sufficiently away from each other and X-rays enter substantially perpendicularly with respect to an X-ray transmissive window, and thus there is only a small influence of a rib on the detection efficiency of X-rays.